The present invention relates to a dual-conductor type current access magnetic bubble memory having transfer gates capable of efficiently transferring magnetic bubbles between the major loop and the minor loops.
The magnetic bubble memory has high storage capacity, is capable of high-speed operation and can store data non-volatilely. Because of these features it is arresting interest as a mass-storage memory. The magnetic bubble memory is essentially a serial access memory which basically comprises a major loop having a port for writing and reading information, a plurality of minor loops for storage of information and information transfer sites (transfer gates) for connection of the major loop to the minor loops.
In general, current-access bubble memories are more highly functional than field-access memories. Among the different kinds of current-access memories, the dual-conductor type memory is receiving the most attention because of the design flexibility it allows.
Magnetic bubble memory technology is fairly well established and is described, for example, in an article titled "Current-Access Magnetic Bubble Circuits" in The Bell System Technical Journal, Vol. 58 No. 6, July-August 1979, pp. 1453-1540, published by the American Telephone and Telegraph Company. Further improvements have been introduced to enhance storage capacity, while at the same time reducing bit cost and bubble driving current. However, there is still room for improvement in the conventional memory in connection with the structure of the transfer gates between the major-minor loops, which is one of particular importance for the magnetic bubble memory. The literature referred to above describes a transfer gate which is so constituted that a relatively small aperture formed in a first conductor is nested within a relatively large aperture formed in a second conductor at one corner of the minor loop to form a gate pattern for bubble transfer. When this arrangement is used, transfer of bubbles between the loops is carried out by modifying the pulse sequence for normal propagation in the loops. For this reason, it is impossible to apply sine-wave current suitable for bubble propagation, so that higher reliability cannot be realized.
To improve the versatility, it has been proposed to partially superpose apertures in the first and second conductors upon each other to form a transfer gate by which the minor loops are connected with the major loop consisting of a straight propagation path having one or two bit-positions each formed by a pair of apertures bored in the first and second conductors. (H. Chang, "Data Structures and VLSI Algorithms for Bubble Chips" IEEE Transactions on Magnetics, Vol. MAG-16, No. 5, September 1980, pp. 764-769). In the straight propagation path known to the art, however, three or more bubbles cannot be transferred continuously between the major loop and the respective minor loops and because of this, the number of information bits transferred cannot be increased beyond one. This and makes it impossible to perform on-chip parity error detection (on-chip parity check) for each minor loop.